This invention relates to glow discharge or plasma reactors and, in particular, to parallel plate reactors.
Initially, in the semiconductor industry, plasma reactors were used for removing photoresist after it had been used to pattern a wafer. Subsequently, plasma reactors became used for a variety of processes formerly carried out with so-called wet chemistries. In addition to these etch processes, deposition processes were also developed. It has remained that the photoresist removal or "stripping" is not so easily done as one might imagine. Photoresist is typically applied to a central spot on a wafer which is then spun to spread the photoresist into a thin layer covering the entire surface of the wafer. Since the vacuum chuck, which holds the wafer during the spin operation, does not cover the entire backside of the wafer, some resist vapors condense on the exposed portions of the backside of the wafer. In addition, a dirty resist spinner chuck will leave a resist residue on the backside of the wafer.
Batch processes support a plurality of wafers in a boat so that there is access to both sides of the wafer, except for where the wafers rest on the rails of the boat. In parallel plate reactors, the wafer rests on an electrode, shielding the backside of the wafer. While one possibility is to add a backside strip to the process sequence, this is undesirable. In general, it is desired to process wafers with as few steps as possible. This not only increases the number of wafers which can be processed per hour, it also reduces handling of the wafers and the chance for breakage and contamination.
At the same time that these process changes were evolving, device structures were also evolving. Specifically, devices were becoming smaller, not only in area but also in thickness. As the layers used to construct a device become thinner, processing become more difficult. Uniformity across a semiconductor wafer is an imperative if all of the devices on the wafer are to have the same operating characteristics. Wafers, meanwhile, are increasing in diameter, making uniformity more difficult to obtain over ever increasing areas.
Another problem which develops as layers become thinner is susceptibility to radiation damage, e.g. what are known as C-V or threshold shift and gate rupture. Thinner layers are less tolerant of radiation than thicker layers. The radiation developed in the plasma is composed of ions, electrons and photons.
The exact constituents of the radiation depend on the gas(es) supplied, the pressure within the reactor, the applied RF power, and the bias applied to the wafer or induced in the wafer from the applied RF power. In terms of charged particles, i.e. the ions in the plasma, the damage depends upon the voltage of the wafer relative to the plasma. Voltages on the order of a few hundred volts are not uncommon. The plasma reactor thus becomes a miniature accelerator. The applied RF power provides a means for exciting the atoms. When the ions revert to a lower energy state, they radiate photons. While the wavelength of the photons can be anywhere in the "light" spectrum, it is frequently in the range of what is known as short ultraviolet, i.e. it is highly actinic.
In view of the of the foregofng, it is therefore an object of the present invention to provide an improved wafer stripping process.
Another object of the present invention is to provide improved plasma processes in which the chance for radiation damage is reduced.
A further object of the present invention is to provide improved plasma processing wherein the plasma has access to both sides of a wafer.
Another object of the present invention is to reduce the number of steps in the processing of semiconductor wafers.